In an increasingly mobile society, portable electric appliances are playing an ever greater role. Rechargeable batteries have been used in virtually all aspects of life for many years. In the development of new types of battery systems, there is particular interest in being able to produce batteries which can be recharged in an inexpensive way and combine a high measure of safety in use with a high specific capacity. In addition, their temperature and shock sensitivity and also their spontaneous discharge rate should be low. Furthermore, a very large number of charging and discharge cycles without losses in capacity should be possible (i.e. high cyclability), as a result of which the product life of the battery can be increased.
The anode of a modern high-energy lithium ion battery nowadays typically comprises graphite, but it can also be based on metallic lithium, a lithium alloy or a lithium compound. The use of lithiated metal oxides such as lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide or in particular lithium vanadium oxide has proven itself in recent years for the production of the cathode of a modern lithium ion battery.
In a lithium ion battery, the two electrodes are connected to one another by means of a liquid or solid electrolyte. Possible liquid electrolytes are, in particular, nonaqueous electrolytes and molten salts. As solid electrolytes, it is possible to use, for example, ionically conductive polymers.
When a lithium ion battery having a cathode comprising a lithiated metal oxide is used (discharge), lithium ions migrate into the layer-like structure of the lithiated metal oxide from which they can be removed again during the charging process. When the battery is discharged, lithium is oxidized at the anode to form lithium ions which then migrate through the electrolyte to the cathode. When a lithium ion battery is recharged, reduction of the lithium ions occurs at the anode. Both during discharge and during recharging of the battery, the lithium ions generally migrate through a separator.
For a battery to be able to be used in the long term, not only the anode and the electrolyte but also the cathode have to have a high chemical and electrochemical stability. Since the ability of the lithiated metal oxides having a layer structure to take up and release lithium ions is of great importance for the stability and also the capacity of the cathode, it is an important task to develop lithiated metal oxides which, as a result of their structure, make long-term reversible migration of lithium ions into and out of the electrodes possible.
Since the crystal structure of the lithium vanadium oxides with relatively low lithium content of the formula Li1+xV3O8 (where x is from 0 to 0.6) was described in detail for the first time about 50 years ago (A. D. Wadsley, Acta Cryst. 1957, vol. 10, p. 261-7), numerous groups of workers worldwide have addressed the use of lithium vanadium oxides for the construction of electrochemical cells. Thus, for example, U.S. Pat. No. 3,929,504 described the structure of a rechargeable battery comprising a lithium anode and an electrolyte material together with a cathode comprising vanadium pentoxide as early as 1975. Later, U.S. Pat. No. 3,970,473 described Li0.33V2O5 and U.S. Pat. No. 5,013,620 described Li1.1V3O8 as cathode materials.
Numerous methods of preparing lithium metal oxides with low lithium content are known. For example, a lithium compound can be heated together with vanadium pentoxide to a temperature of about 680° C. to give a fused mass which can subsequently be ground to a powder (S. Pistoia et al., Solid State Ionics 13 (1984), pages 311 to 318).
U.S. Pat. No. 5,520,903 describes a process for preparing lithium vanadium oxides with low lithium content in which a lithium compound such as lithium hydroxide and a vanadium compound such as vanadium pentoxide are mixed, subsequently pressed and then heated to a temperature of at least 570° C.
The US patent application 2005/0026041 describes lithium vanadium oxides which are prepared by pulverizing vanadium oxide and lithium carbonate and subsequently calcining the mixture at 580° C. for 10 hours. This document also describes the construction of a lithium ion battery and the testing of the cathode stability.
Many further processes for preparing lithium metal oxides with low lithium content which comprise the main process steps of mixing of the components, comminution or milling of the intermediate obtained and subsequent calcination are known. However, owing to the high calcination temperatures used, these processes are unsuitable for preparing lithium-rich metal oxides which are frequently thermodynamically unstable.
The preparation of lithium-rich metal oxides is described, for example, in U.S. Pat. No. 5,980,855. The process comprises reacting a metal oxide with lithium metal in an organic solvent in the region of room temperature in the presence of an aromatic hydrocarbon as catalyst.
The preparation of lithium-rich vanadium oxides by reaction of a vanadium oxide with low lithium content with n-butyllithium in n-hexane at room temperature is described by J. Kawakita et al. in Solid State Ionics 118 (1999), pages 141 to 147.
U.S. Pat. No. 6,083,475 discloses the preparation of lithiated metal oxides by reaction of a metal oxide with lithium sulfide in an organic solvent under reflux. The solvent is preferably selected so that it dissolves both the lithium sulfide and the sulfur formed while the metal oxide and also the lithiated metal oxide are not dissolved.
All these processes can be carried out on an industrial scale only with great difficulty or lead to process products which are not suitable for producing high-performance and durable cathodes.